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Geological Origins of Cardiff

Long before man set foot on the earth, mighty forces of nature shaped the landscape which he would inherit and exploit. Over 500 million years ago, the region we know as South Glamorgan was covered by a warm, shallow sea. About that time volcanic eruptions began a process which was to deposit the oldest rocks of Glamorgan in the Penylan district of Cardiff. Though more than 400 million years old, these stones appear relatively young when we remember that their age is a mere tenth of the most ancient rocks known to man.

Since those first volcanic disturbances, the action and interaction of sun, wind, rain, frost and ice have been at work to create the natural environment of South Wales. First came a lengthy period when vast accumulations of sand from distant deserts, aided by a warm wet climate, created the red sandstone which is a feature of the Brecon Beacons to this day.

Conditions changed and a tropical sea covered the area. Evidence of a clear coral lagoon has been discovered at Barry Island where the sediment provided a fertile environment for molluscs, corals and sea lilies. Eventually this silt came to the surface as carboniferous limestone, a rock which is still visible in the gorge at Taffs Well, on the islands of Flatholm and Steepholm, and in the Vale of Glamorgan. For many years this limestone has been quarried at Wenvoe and much of the material used in the construction of Barry Docks came from here.

About 300 million years ago the sea level fell away to produce a marshy plain, criss-crossed by innumerable streams. The climate was humid and tropical, turning the plain into a swampy jungle. As the lush, rotting  vegetation was buried under a layer of mud and sand, rich seams of coal were formed. In the Vale of Glamorgan they eventually became eroded and disappeared, but to the north these seams evolved as the South Wales coalfield. Thus was created the mineral wealth which brought prosperity to Victorian Cardiff and the mining valleys.

Yet another lengthy period of desert-like conditions followed, as sand and dust were blown together to shape Triassic sandstones and marls. They can be seen along the coastline from Penarth to Barry, and the attractive Radyr stone is also a legacy of this period. The stones in the Gorsedd circle at Cathays Park are natural examples of Triassic sandstone, and much of the material used to build Victorian Cardiff came from this source.

During this Triassic era, dinosaurs roamed the earth. In 1974 the footprints of one of these monsters was found near Porthcawl and in 1990 more extensive impressions were discovered near the Atlantic Trading Estate at Barry. Preserved under a mixture of mud and sand, some of these footprints are thought to have been made by the Coelophysis. This was a strange beast, approximately four feet high and measuring about eight feet from head to tail. In appearance it resembled a kangaroo, walking on its hind legs to leave a distinctive image of its three toes in soft mud. These fossils from the age of the dinosaurs can be seen in the National Museum of Wales.

Once more the sea flooded the land, bringing in its wake the shale and Liassic limestone so characteristic of the Vale of Glamorgan. In time this limestone would be used to build Cardiff Castle, St. John's Church, Llandaff Cathedral and many churches in the Vale itself. The limestone also produced a light but fertile, stony soil which could be cultivated with the primitive tools of the earliest farmers.

One more great geological transformation was to occur. The Ice Age began about a million years ago and finally terminated a mere 10,000 years from our own lifetime. Massive glaciers covered most of South and Central Wales. As they melted in the fullness of time, so the sea level rose and the future English and Bristol Channels at last made their appearance. The melting glaciers left a fine debris which became the basis of our clay and gravel soils. Much of Cardiff is built on the deposits of this final stage of creation.

 

Further Research:

 

Prehistory galleries at The National Museum of Wales

North F.J. “The Geological History of the Vale”, in “The  

           Garden of Wales”, ed. Stewart Williams (Barry

           1961)